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SteveGerber

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Posts: 27
Reply with quote  #11 
Ok, I added a second temperature probe on the opposite wall inside the room. I change the differential logic so that the fan turns on when the collector temperature reaches (room temperature + 15) and shuts off when the collector temperature falls to (room temperature + 5)

Any thoughts regarding these ranges? In particular, how close together do you think the on/off trigger points can be before I would get excessive on/off cycling? At the moment, as you can see, they are 10 degrees F apart.

By the way my collector temperature probe is currently located toward the top of the collector just below the fan but behind the double screens, so the probe is not receiving direct sun light.

Currently: 10:40 a.m. on December 21
Latitude: 38.5 degrees
Harrisonburg, Virginia
Collector temperature: 85F
Room temperature: 68F
Outdoor temperature in shade: 36F

I've attached a photo of my collector. It still needs some finishing touches but will give you an idea what I'm working with. It's sloped at about 30 degrees from vertical. It's actually heating two side by side rooms through separate sets of intake and output vents. Each room has it's own 120mm computer fan but right now both fans are connected to the same relay. Without much more effort I could add another temperature probe to the second room and control it's fan with a different relay.

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jpeg IMG_20171221_102304.jpg (370.33 KB, 37 views)

gbwillson

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Reply with quote  #12 
Steve-

Interesting design heating two separate rooms with the same collector. Almost looks like if you opened up the wall to the house you could use it as a sunny place for plants. As deep as your collector box is I was wondering about the benefit if the side walls were clear. I suspect it would heat up earlier, and stay running later.

Most of the preset snap switches I've seen have a 15˚F differential. Some control units have the ability to set a delay to reduce excessive cycling. Any incoming air warmer than the ambient room air is adding BTU's to a home. But a lukewarm breeze may make a room seem drafty, so you need to pay attention to the location of the output air. While subjective, 85˚F incoming air seems rather cool unless your hand is directly in from of the output. So you may want to consider bumping the upper set point 5˚-10˚ degrees to a comfortable temperature. 

I know you aren't quite finished with the collector, but I see a couple of possible issues. Is the black backing with the writing on it tar paper which was used for decades as an underpayment for siding? If so, you will likely have an unpleasant smell as the unit heats up. 

The second thing I see is what looks like blue XPS, or extruded polystyrene. XPS can't handle the heat and can melt as low as 180˚F, which only takes a few minutes to reach if the fans stop working for some reason.

Greg in MN


SteveGerber

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Posts: 27
Reply with quote  #13 
Thanks for the advice Greg! So far I only have metal on the bottom but I plan to eventually cover all the internal sides with metal. I too was concerned that the sheathing might smell bad (it's that old style 1/2 inch (cardboard?) stuff that seems to have a light tar coating, but so far no odor maybe because it hasn't gotten all that hot in the box. If the fans stop working it should still vent hot air into the rooms but it would be interesting to turn the fans off on a sunny day and see how hot the collector gets if it just thermosiphons through the relatively small vents past the fan blades.

I understand your point about blowing cool feeling air around that is only slightly warmer than the room temperature, but if the rooms are unoccupied during the daytime then it doesn't really matter. One room hit 70F around 11:30 a.m. and I decided to open the door and let it spill heat out into another large colder part of the house. I was out for several hours and when I got back home around 3:00 p.m. the collector was still running and the room temp was down to 63F (due to spilling heat into the hallway) and the collector probe was at 76F. I didn't see exactly when it quit running but I'd estimate it was probably around a collector temperature of 68F (room temp + 5). Like you said, blowing cool air, like 68F [frown] onto somebody wouldn't feel so good even if it is technically warmer than the ambient room temperature, but I was certainly extracting a lot of extra BTUs beyond what I was getting when I was using the 85F-100F snap disc. This was the first totally sunny day I had to test it and I can get at least a little heat starting around 9:30 a.m. and ending around 3:30 p.m. on the shortest day of the year.
gbwillson

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Reply with quote  #14 
Steve-

I kinda thought that looked like the "tar-impregnated cardboard" insulation on my old house that I removed when I re-sided my house. Yes, you WILL get a smell unless you can seal it/cover it. Not sure if there are possible health issues breathing air exposed to the surface. In any case, it's something to consider...

Once you have had a few cold, sunny days hopefully you can experiment with different settings if adjustment isn't too difficult. And 6 hours of run time at this time of year is really good. And runtimes will only get longer and performance stronger for months to come.[smile]

Greg in MN
SteveGerber

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Posts: 27
Reply with quote  #15 
This morning I made another change to my system. I realized that since one of the two rooms connected to my solar air heater is a guest room that is rarely used at all I might as well turn that fan on independently and at a temperature that is even closer to that room temperature. This guest room is left unheated when not in use and temperatures can fall as low as the upper 40s on a really cold night so it could benefit from even quite low collector temperatures.

So I added a third temperature probe on the opposite wall of the guest room and connected the fan for that room to it's own relay. This allows me to control the rooms independently as well as set separate temperature differential ranges for each room. The guest room is currently set to turn on the fan at (room temperature + 8) and off at (room temperature + 3). So in theory, this should enable the unoccupied guest room to collect BTUs even under very marginal conditions.

This morning the sky is lightly overcast but I can tell where the sun is at.
Collector temperature: 64F
Guest room temperature: 53F
Ambient outdoor temperature: 46F

So the fan is running in the guest room but the fan is not running on the more frequently used (and heated) middle bedroom which is still set at: on at +15 and off at +5.
SteveGerber

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Posts: 27
Reply with quote  #16 
I added a SD card to my Arduino microcontroller so that I could log temperature changes and when the fans come on. I wasn't sure whether it would be best to only log changes or if I should just collect data on a fixed interval. Right now I'm simply logging data once per minute. The drawback to this is that I might not get as many data points as I'd like if a rapid change occurs, but I guess that's not too likely with a hot air panel. Also I may generate piles of data when nothing is happening. On the other hand, if I just log changes, it may not give a very accurate picture of what happens over time since there may be long periods with no change which would get compressed misleadingly on a graph.

My current record format is:
collectorF,room1F,room2F,fan1,fan2
for example:
63,70,56,0,0

So it's a pretty small record size, around 13-14 bytes, so it shouldn't add up too fast. Might be nice to add a fourth probe and track outdoor temperature as well.

Any thoughts about data collection intervals or other useful data to collect?
guy

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Posts: 27
Reply with quote  #17 
I have 4 temperature probes that I record with collector output,room,outside and near a furnace register it shows up as blip on my graph if the furnace comes on. I log temp data once per minute and have seen temperature changes of 15 to 20 degrees in one minute. For recording rapid changes in temperature you would want as small a probe as possible to reduce temperature lag. The probe I use will fit through the holes in a screen. I also have a hour meter hooked to my fan to record heating hours for the day,month and season.    P1030027.jpg 
gbwillson

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Reply with quote  #18 
Steve-

Take a look at how Jake logs data for his two collectors. I think if you are going to log data the two most important probe locations would be the intake and exhaust of the collector box. Other locations might be outdoor ambient air temp, temp at entry to home etcetera. Jake knows the output CFM for each collector, so he automatically calculates the BTU's. 

I really like how he calculates the cost of electricity used to run the fans, but also his savings over using electricity for heating. I do wish he posted more than three days worth of data. I'd like to see a more detail and 3-4 weeks worth of data to see trends over time. 

http://simplysolar.supporttopics.com/post/jakes-solar-north-of-50-9446061?pid=1301582722

http://jeharms.ca/cottage/index.php

Greg in MN
SteveGerber

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Posts: 27
Reply with quote  #19 
Thanks for the responses! I might have to buy some more temperature sensors. [smile] I can see how it would be useful to monitor intake and output air temps to see how much increase I'm getting. I think I'll probably also order a real time clock module for the Arduino so I can time stamp the log entries.
SteveGerber

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Posts: 27
Reply with quote  #20 
My solar hot air collector has been working pretty well over the past week.  Even with outdoor temperatures around 15F-20F it still makes a useful amount of heat for the two rooms it's connected to.  Overnight the outdoor temps dropped to around 2F and the unheated spare room fell to 41F.  This morning is a nice clear sunny day and the collector first turned on at 9:05 a.m. and started giving that room 52F degree air.  The fan for the occupied and heated room (67F) came on about a half hour after that once the collector had reached 77F.

Right now at about 11:10 AM the collector registers 90F (probe is behind screens and both fans are running.) The spare room is up to 55F and the occupied room is back up to 68F and rising (after about a 4 degree drop off when the electric heat shut off).  My outside probe registers 26F but that's deceptive because it's under the collector on the south side of the house.  A different indoor/outdoor thermometer reads 16F because it's outdoor sensor is in the shade on the west side of my house.  I'm not quite sure what temperature I should actually be recording with my data logger as the true outdoor temperature.  The probe on the south side seems inaccurately high compared to the true ambient temperature in the shade, but on the other hand the collector is attached to the south side of the house and is operating within that warmer microclimate bubble.

This first solar hot air collector project is giving me a much better awareness of how many interacting variables there are and how tricky it can be to get a true reading on how the panel performs as conditions vary.  The ambient temperature, cloudiness, angle of sun on the glass, starting room temperature, insulation value of the room, whether or not the room window is contributing heat or losing heat and even some partial shading from the very top of a tall tree two houses over that lowers insolation detectably for about 30 minutes at around 11:45 P.M.  Fortunately the sun will soon rise completely back above that tree in a week or two.  It doesn't have a huge affect but I can definitely  see the collector temps drop off as the sun pass through the very top bare branches of the tree.  I also placed 12 square feet of mirrors almost flat below the collector yesterday to see if I could detect a boost from them, so that's yet another variable.  At any rate, it's definitely pumping out warm air during some really cold weather and that's encouraging!  [smile]

I'm definitely learning a lot from actual monitoring of a real device attached to my house which will help me make decisions about the design and location of an future solar systems that I construct.  [smile]
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